preload: Record the metrics of triggered mousedown and mouseover

This CL introduces PreloadBookmarkMetricsEvent to evaluate
the percentage of these two events translating into a full click.

Bug: 1422819
Change-Id: Ib653c5cbe5c01319700c0924b0e13aae06640226
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/4322977


Reviewed-by: Kouhei Ueno <kouhei@chromium.org>
Commit-Queue: Huanpo Lin <robertlin@chromium.org>
Reviewed-by: Takashi Toyoshima <toyoshim@chromium.org>
Reviewed-by: Scott Violet <sky@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1118501}